136 Fundamentals of Auxin Action 



and less for phosphorus and nitrogen. Furthermore, the potassium 

 changes were very much in the same order as the decrease in dry 

 weight. On the basis of their data they suggest that toxic effects of 

 MCPA may be exerted through some effect on potassium availability. 



While herbicidal applications of auxins may mobilize nitrogenous 

 materials in some parts of the plant, the uptake of nitrate by the plant 

 may be inhibited. Nance (1949) has found that the uptake of nitrates 

 by wheat roots is drastically inhibited by small concentrations of 2,4-D. 

 It is possible that the reduction in nitrogen content of tomato plants 

 (shown in figure 62) may be caused by the inhibition of this function 

 in the roots. This inhibition of nitrate absorption is not associated 

 with any observable change in oxygen consumption and the inhibition 

 is reversed by supplying small amounts of citrate. These observations 

 led Nance to suggest that 2,4-D may selectively block the anion respi- 

 ration mechanism described by Lundegardh (1947). 



An interesting effect of lethal doses of 2,4-D on the inorganic 

 phosphate content of bean plants was noted by Loustalot and Morris 

 (1953). They found that treatinent with 1,000 mg./liter of 2,4-D nota- 

 bly increased inorganic phosphate as the plants approached death. 

 They suggest that, since the accumulating phosphate must originate 

 in the organic constituents of the plants, the herbicidal result may be 

 effected through interference with normal phosphorylation reactions. 



In conclusion, it must be reiterated that the application of auxins 

 to plants can residt in a host of complex interactions. It has been 

 pointed out that auxins may have great effects on many developmental 

 growth functions including tropisms, inhibitions, morphological dif- 

 ferentiation, and flower and fruit development. Auxins can bring 

 about a great variety of anatomical effects, including cellular prolifer- 

 ation, mitotic irregularities, and epinastic deformities. Auxins can 

 bring about large changes in the metabolism of the plant as reflected 

 in gross changes in respiration, the activity of various enzymes, in 

 the abundance of various common plant constituents, and in the 

 availability of some ions to the roots. Theories as to how this immense 

 diversity of responses may be brought about are discussed in chapter 

 VIII. 



